Visible-Light-Induced Passerini Multicomponent Polymerization

Article abstract of DOI:10.1002/anie.201901506

Herein, we introduce an additive-free visible-light-induced Passerini multicomponent polymerization (MCP) for the generation of high molar mass chains. In place of classical aldehydes (or ketones), highly reactive, in situ photogenerated thioaldehydes are exploited along with isocyanides and carboxylic acids. Prone to side reactions, the thioaldehyde moieties create a complex reaction environment which can be tamed by optimizing the synthetic conditions utilizing stochastic reaction path analysis, highlighting the potential of semi-batch procedures. Once the complex MCP environment is understood, step-growth polymers can be synthesized under mild reaction conditions which—after a Mumm rearrangement—result in the incorporation of thioester moieties directly into the polymer backbone, leading to soft matter materials that can be degraded by straightforward aminolysis or chain expanded by thiirane insertion.

Full text of DOI:10.1002/anie.201901506

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Visible Light-induced Passerini Multicomponent Polymerization
Authors: Bryan Tuten, Lies De Keer, Sandra Wiedbrauk, Paul Van
Steenberge, Dagmar D'hooge, and Christopher Barner-
Kowollik
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201901506
Angew. Chem. 10.1002/ange.201901506
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10.1002/anie.201901506
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Visible Light-induced Passerini Multicomponent Polymerization
Bryan T. Tuten,[a] Lies De Keer,[a,b] Sandra Wiedbrauk,[a] Paul H.M. Van Steenberge, [b]
Dagmar R. D’hooge,*[b,c] Christopher Barner-Kowollik*[a, d]
Abstract: Herein, we introduce an additive free visible light-induced
Passerini multicomponent polymerization (MCP) for the generation of
high molar mass chains. In place of classical aldehydes (or ketones),
highly reactive, in situ photogenerated thioaldehydes are exploited
along with isocyanides and carboxylic acids. Prone to side reactions,
the thioaldehyde moieties create a complex reaction environment
which can be tamed by optimizing the synthesis conditions utilizing
stochastic reaction path analysis, highlighting the potential of semi-
components have advantages in that they lead to unique
functional moieties beyond the typical amide and ester linkages
in a single step that would otherwise require multiple (indirect)
steps to access.
Herein, we pioneer an additive free, visible light-induced Passerini
multi-component polymerization (MCP, Scheme 1) utilizing highly
reactive, photo-caged thioaldehydes (C)
– derived from
pyreneacyl sulfide derivatives (CDx) – as the carbonyl component
batch procedures. Once the complex MCP environment is understood, instead of the classically utilized aldehydes and ketones.
step-growth polymers can be synthesized under mild conditions,
which – after a Mumm rearrangement – result in the incorporation of
thioester moieties directly into the polymer backbone, leading to soft
matter materials that can be degraded via straightforward aminolysis
or chain expanded via thiirane insertion.
Thioaldehydes have long been known as powerful ligation
moieties, yet due to their highly reactive nature are often
challenging functional groups to access.[9] Recently, our team has
synthesized and fully mapped a visible light-induced thioaldehyde
via a pyreneacyl sulfide ligation platform that is accessible in a
single step and triggered under very mild conditions (atmosphere,
visible light, and ambient temperature).[10] We have also
incorporated pyreneacyl sulfides via an Ugi four part
multicomponent reaction accessing fluorescent read-out of spin-
silenced systems.[11] These highly reactive electrophiles have
been shown to undergo a broad range of reactions including
hetero Diels-Alder cycloadditions, nucleophilic trapping with
amines and alkoxyamines, disulfide formation, and 1,3-dipolar
cycloadditions.[12] In the present work, we critically expand the
horizon of the thioaldehyde’s ligation toolbox into a mild, visible
light-induced Passerini MCP reaction, a key advance in visible
light-induced polymerizations.[13]
Multicomponent reactions have become one of the most powerful
and valuable tools in organic and polymer synthesis. Based on
their broad scope of applications ranging from natural product
synthesis, drug design, chromophore synthesis, and even
cryptography.[1] Recognizing the versatility and usefulness of
these powerful ligation tools, polymer chemists have incorporated
them into their field where they have become permanent fixtures.
Multicomponent reactions have enjoyed success in the polymer
field for post polymerization modification,[2] sequence control,[3]
and direct step-growth polymerization.[4]
Recently, there has been interest in using ‘non-traditional’
partners in multicomponent Passerini and Ugi reactions including
as acid component thioacetic acids[5] and hexafluoro
isopropanol,[6] and as carbonyl component ketenes[7] and TiCl4
mediated thiocarbenium species.[8] These ‘non-traditional’
Polymers featuring thioester moieties within their backbone have
recently garnered intense attention, highlighted in the pioneering
research by Bowman and coworkers, who showed that these
functional handles act as dynamic covalent crosslinking sites in
polymer networks, able to selectively toggle between flowing and
solid states by photo acids/bases.[14] The advantage of
photogenerated thioaldehydes in a Passerini MCP rests in the
final Mumm rearrangement, allowing thioesters to be directly
incorporated into the polymer backbone of the growing chain,
bypassing the need to first synthesize a thioester-containing
monomer. Having polymers with thioester linking motifs provide a
unique route towards directly erasing the polymers via simple
aminolysis or chain expansion by selective insertion
polymerization of thiiranes.[15]
Due to the high reactivity and short lifetimes of photo-generated
thioaldehydes as well as the complex nature of photo-induced
MCPs, an in-depth understanding of the relation between
synthetic conditions as well as product yield and purity is critical
in order to successfully develop a robust visible light-induced
MCP methodology.[16] Our research team has recently
demonstrated that qualitative stochastic or kinetic Monte Carlo
(kMC) screening is indispensable in order to understand complex
reaction mechanisms and to control selectivity toward a targeted
product.[17] The need for kMC modelling of Passerini MCPs
became immediately apparent upon our initial attempts at using a
[a]
[b]
[c]
[d]
Dr. B. T. Tuten, Dr. S. Wiedbrauk, Prof. Dr. C. Barner-Kowollik
School of Chemistry, Physics and Mechanical Engineering
Queensland University of Technology
2 George Street, QLD 4000, Brisbane (Australia)
E-mail: christopher.barnerkowollik@qut.edu.au
L. De Keer, Dr. P.H.M. Van Steenberge, Prof. Dr. D.R. D’hooge
Laboratory for Chemical Technology
Ghent University
Technologiepark 914, B-9052 Ghent (Belgium)
E-mail: dagmar.dhooge@ugent.be
Prof. Dr. D.R. D’hooge
Centre for Textiles Science and Technology
Ghent University
Technologiepark 907, B-9052 Ghent (Belgium)
E-mail: dagmar.dhooge@ugent.be
Prof. Dr. C. Barner-Kowollik
Macromolecular Architectures, Institut für Technische Chemie und
Polymerchemie
Karlsruhe Institute of Technology
Engesserstrasse 18, 76128 Karlsruhe (Germany)
E-mail: christopher.barner-kowollik@kit.edu
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COMMUNICATION
Scheme 1. Proposed mechanism of visible light-induced Passerini multicomponent step-growth polymerization utilizing photogenerated thioaldehydes (blue, Cx),
diisocyanide (fucia, BB) and dicarboxylic acid (green, AA) monomers. The targeted product has a repeat unit AACBBC; no side reactions shown.
benzyl pyreneacyl sulfide (CD1) monomer, a diisocyanide (1,6-
diisocyanohexane, BB) monomer, and a diacid monomer (AA) in
one pot under blue LED irradiation (415-430 nm) to initiate batch
polymerization. After several attempts at polymerizing all three
components, only a limited number (e.g. 1 or 2) successful
Passerini linkages were observed, with a Mw of, maximally, 900 g
for the number average chain length (xn) up to high functional
group conversion (Figure 1A, red simulated line). To maximize the
selectivity of the thioaldeyde-based Passerini reaction, according
to theory, a semi-batch approach is required ensuring that the
excess of diisocyanide and diacid in relation to the photo-
generated thioaldehydes moiety is maximized. As evident in
Figure 1A (simulated green line), such an approach leads to a
step-growth polymerization environment which follows Carother’s
equation up to very high functional group conversion, assuming
that the intrinsic reactivity of side reactions is sufficiently low
compared to the desired Passerini reactions (Figure S11). Thus,
we added the diacid (AA) and the diisocyanide (BB) monomers
mol-1 (Mn of 600
g
mol-1) according to size exclusion
chromatography (SEC, polystyrene standards; green line in
Figure 1C). Qualitative batch kMC simulation (refer to Section 3
of Supporting Information) demonstrates that di- and trimerization
side reactions of the C moieties, as well as oligomerization of the
thioaldehydes in combination with the ability of the acetyl pyrene
byproduct (D) to also take part in a Passerini reaction, out-
compete the desired thioaldehyde-mediated Passerini
polymerization mechanism affording Px (Scheme 1; n >> 1). The
excess of undesired side reactions, when modelled, clearly
indicates that batch conditions do not follow Carother’s equation
A)
B)
C)
Batch
Side reaction
Side reaction
Semi-batch (addition of CD2)
Semi-batch (addition of AA)
.
.
Batch:
.
Batch:
[AA]0:[BB]0:[CD]0 = 1:1:2
[AA]0 = 0.48 mol L-1
Semi-batch AA:
AA added at 5.6 10-7 mol s-1
for 6 hours
[AA]0:[BB]0:[CD]0 = 1:1:2
[CD]0 = 0.97 mol L-1
Semi-batch CD:
CD added at 1.1 10-8 mol s-1
for 6 hours
.
[BB]0:[CD]0= 1:2
[AA]0:[BB]0= 1:1
24
26
28
30
32
34
elution time [min]
Figure 1. A) Qualitative kinetic Monte Carlo (kMC) simulation results (details in the Supporting Information) for the number average chain length xn under batch
([AA]0:[BB]0:[CD]0=1:1:2) and semi-batch conditions with addition of CDx, including a formal side reaction with only C. B) Qualitative kMC simulation results for xn
under batch ([AA]0:[BB]0:[CD]0=1:1:2) and semi-batch conditions with addition of AA, including a formal side reaction between AA and C.. Critically, in both A)
and B) only with semi-batch conditions is it theoretically possible to achieve high functional group conversion, provided that the side reaction reactivities are not
too high. C) Experimental results demonstrating the size exclusion chromatography (SEC, RI detector) traces under batch conditions (green; limited
polymerization), semi-batch conditions for addition of CD2 (magenta; limited polymerization), and semi-batch conditions for AA (blue; significant polymerization,
P1), due to preformation of the nitrilium ion species CBBC. (Scheme 1).
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Label m/z (exp) m/z (theo)
m/z
D
1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
1749.7171 1749.7120 0.0051
1763.6779 1763.6730 0.0049
1781.6878 1781.6841 0.0037
1835.7722 1835.7674 0.0048
1851.7663 1851.7618 0.0045
1867.7438 1867.7395 0.0043
1935.8070 1935.8021 0.0049
1953.8165 1953.8127 0.0038
1969.7926 1969.7898 0.0028
2211.9256 2211.9221 0.0035
2227.9032 2227.8993 0.0039
2245.9128 2245.9099 0.0029
2466.0524 2466.0497 0.0027
2484.0626 2484.0602 0.0024
2502.0718 2502.0708 0.0010
2
4
3
5
7
6
8
10
11
12
13
14
15
9
1800
2000
2200
2400
2600
m/z
Figure 2. Size exclusion chromatography-electrospray ionization-mass spectrometry (SEC-ESI-MS) spectrum of P1 ranging from 1700 to 2700 m/z in negative ion
mode with sodium iodide (NaI). All repeat units (and their corresponding end-groups) over 20% relative abundance were assigned. Structure 11 is displayed as a
representative example. For structural assignments refer to Figure S8 in the Supporting Information.
directly into the reaction vessel, while dissolving the thioaldehyde
precursor (CD1) in a minimum volume (0.5 mL) of solvent and then
utilized a syringe pump to slowly, over the course of 2 hours, add
CD1 into the irradiated flask containing AA and BB. By slowly
adding the photomonomer a semi-batch synthesis recipe is
followed, as identified as the optimum reaction setup by kMC
modelling. However, as evidenced with an advanced hyphenated
experimental technique, size exclusion chromatography-
electrospray ionization mass spectrometer (SEC-ESI-MS), the
number of photo-induced side reactions when utilizing C1 (di, tri-,
and oligomerization), still disallows the formation of a polymer
containing predominantly Passerini-based linkages. Thus, the
intrinsic reactivities of the side reactions with C1 are higher than
that of the desired Passerini mechanism, demonstrating it is still
a non-ideal system as predicted by our first qualitative predictions.
We utilized SEC-ESI-MS in order to take a slice-by-slice view of
the polymer, allowing insights into the internal macromolecular
structure of these Passerini polymers, as well as the end-group
patterns in these new visible light-induced Passerini step-growth
polymers (Figure and Figure S8 of the Supporting
2
Information).[18] For CD1, the number and rate of photo-induced
side reactions are out-competing the desired MCP (magenta line
in Figure 1C), and thus we designed a second photo-generated
monomer which leads to far fewer side-reactions, implying lower
intrinsic rate coefficients for side reactions upon thioaldehyde
generation.
Thus, we explored a neopentyl-based thioaldehyde derivative
(CD2), as these sterically bulkier, photo-generated thioaldehydes
are far more stable, and in some instances, even stable in
solution.[19] These neopentyl-based thioaldehydes have been
shown to exhibit a far lower propensity to di- and trimerize, as well
as minimize oligomerization when compared to the
thiobenzaldehyde-based monomer.[20] After submitting the new
neopentyl pyreneacyl sulfide monomer to both CD2 semi-batch
conditions and AA semi-batch conditions (two separate
experiments), we found the AA semi-batch conditions to be
Scheme 2. Schematic representation of P1 being deleted via aminolysis (n-butylamine at ambient conditions overnight) of the thioester linking motif (right hand
pathway) as well as chain expansion via selective thiirane insertion (propylene sulfide at 80 oC in N,N’-dimethylacetamide for 18 hours with tetrabutylammonium
chloride as catalyst) into the thioester linking motiff (left hand pathway).
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SEC-ESI-MS provides further support of the updated mechanism
of the Passerini reaction proposed by Rammozi and
Morokuma.[22]
Mw = 12300 g mol-1 (Ð=2.88)
Mw = 6030 g mol-1 (Ð=1.77)
Mw = 1290 g mol-1 (Ð=2.02)
Exploiting optimized reaction parameters with CD2, we proceed to
highlight the versatility of a Passerini polymer with an internal
thioester moiety (Scheme 2). The soft matter material was readily
deleted (right pathway in Scheme 2) via a simple and mild
aminolysis procedure which only entails stirring in the presence of
n-butyl amine overnight at ambient temperature. Evident in the
SEC traces depicted in Figure 3, (orange line, Mw 1290 g mol-1,
Mn 640 g mol-1), there are remaining trace amounts of oligomers.
This is attributed to a very small number of ester linkages instead
of thioester linkages caused by photooxidation of C2 into
neopentyl aldehyde during polymerization (Figure S8, structures
5 and 10).[23] We have also analyzed the degradation products via
high resolution ESI-MS (Figure S9) where the two primary
degradations products (Scheme 2, right side) can clearly be
identified. While P1 can be deleted, it can also undergo chain
expansion via the selective insertion of propylene sulfide into the
thioester moiety (Figure 3, Scheme 2, left pathway, Figure S9).
By using tetrabutylammonium chloride (TBAC) as catalyst in
dimethylacetamide (DMAc) at 80 oC we were able to insert
poly(propylene sulfide) segments directly into P1, more than
doubling the Mw to 12,300 g mol-1 (Mn 4270 g mol-1, black line,
Figure 3) forming P2.
In summary, we have demonstrated that through the synergistic
combination of both experimental as well as modelling
methodologies, a complex yet powerful new visible light-induced
Passerini multicomponent polymerization can be established. We
show that reactive photo-generated thioaldehydes undergo
Passerini reactions in lieu of traditional aldehydes, resulting in a
polymer with direct thioester incorporation. As predicted by kMC
simulations, semi-batch operation minimizes undesired side
reactions during MCP, affording the desired Passerini polymers.
Critical for visible light-induced MCPs is the importance of
selecting a photo-caged thioaldehyde which undergoes minimal
di-, tri-, and oligomerization. Finally, we have demonstrated that
thioester containing Passerini polymers can readily be deleted or
expanded by exploiting the unique properties of thioester moieties.
The current work breaks new ground in utilizing mild, visible light
to combine the powerful Passerini reaction with the versatility of
thioaldehydes to create functional soft matter materials.
22
24
26
28
30
32
34
elution time [min]
Figure 3. SEC elugram (RI detector) of P1 (blue trace; starting from CD2)
obtained via utilizing a semi-batch procedure with addition of AA, after
aminolysis (orange trace) and after thiirane insertion, P2 (black trace), cf.
Scheme 2.
superior. We submit that the primary culprit to low P1 yield under
the CD2 semi-batch conditions is the thioaldehydes (C) being
generated in the presence of an excess of carboxylic acid (AA)
which thereby destroys them.[21] Indeed, with such a side reaction
being formally active, as illustrated in Figure 1B, AA semi-batch
conditions are theoretically needed to allow for a high functional
group conversion. AA semi-batch conditions are further
supported by Ramozzi and Morokuma’s intriguing work into the
nature of the Passerini reaction mechanism. According to these
authors, the nitrilium ion intermediate (CBBC in Scheme 1) should,
in principle, be stable in solution.[22] Thus, the photoreactive
monomer (CD2) and the diisocyanide monomer (BB) should be
present at the start of the MCP instead of BB and the carboxylic
acid (AA). Under blue light irradiation, AA is added into the
reaction vessel containing CD2 and BB (over 2 hours via syringe
pump; see Figure S1 for details). Thus, AA preferentially reacts
with the pre-formed nitrilium ion (CBBC, Scheme 1) instead of
immediately destroying C, which in turn would completely inhibit
the polymerization. The efficacy of this method (pre-formation of
the nitrilium ion) is evident with the immediate increase in average
molar mass (Figure 1C, Mw 6030 g mol-1, Mn 3440 g mol-1; blue
line) as well as a complete prohibition of acetyl pyrene
incorporation (via the ketone moiety) into the polymer backbone,
yielding polymer P1. The alternative semi-batch conditions were
also applied with CD1, however, the rate of side reactions for CD1
still proved too high for successful Passerini MCPs.
Experimental Section
All relevant experimental information as well as supplementary data are
included in the Supporting Information.
Once again, to gain a detailed structural overview of our Passerini
polymer, we conducted SEC-ESI-MS, which indicated very few
side reactions as well as no observable incorporation of acetyl
pyrene into the polymer backbone (Figure 2). Furthermore, we
confirm the high number of combinations of various end-groups
of the Passerini MCPs. A few end-group combinations of the
polymers include unreacted nitrillium ion species (Figure S8,
structures 7 and 8). We submit that the increase in molar mass
via the AA semi-batch method of pre-forming the nitrilium ion
species as well as the results obtained in end-group analysis via
Acknowledgements
C. B.-K. gratefully acknowledges funding from the Australian
Research Council (ARC) in the form of a Laureate Fellowship,
underpinning his photochemical research program as well as key
support from the Queensland University of Technology (QUT). D.
R. D. acknowledges the Research Foundation-Flanders (FWO,
G0H5216N). B. T. T. thanks K. Hosokawa (QUT) for continued
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Hampshire) for fruitful discussions. L. D. K. acknowledges support
by the FWO through a SBO project (1S37517N, 2017-2018). P.
H. M. V. S. acknowledges the FWO through a postdoctoral
fellowship.
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Keywords: multicomponent chemistry • Passerini reaction •
photochemistry • kinetic Monte Carlo screening • step-growth
polymerization
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Entry for the Table of Contents
COMMUNICATION
Lights on for Multiple Components,
Bryan T. Tuten, Lies De Keer, Sandra
Wiedbrauk, Paul H. M. Van Steenberge,
Dagmar R. D’hooge,* Christopher
Barner-Kowollik*
Multiple
Functions:
Passerini
multicomponent polymerizations can
be triggered with mild visible light
utilizing photo-caged thioaldehydes.
The complex, high reactivity of
thioaldehydes is tamed via stochastic
kinetic Monte Carlo simulations
yielding highly functional soft matter
materials capable of being degraded
as well as chain expanded.
Page No. – Page No.
Visible Light-induced Passerini
Multicomponent Polymerization
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